1. Field of the Invention
The present invention relates to a fluidized bed type pulverization/classification apparatus for manufacturing a powder, which may particularly be a toner powder, and a method of manufacturing the powder using the pulverization/classification apparatus.
2. Discussion of the Background
Background fluidized bed type pulverization/classification apparatuses typically have a cylindrical shape and include a vessel. In addition, plural air nozzles are provided on lower portions of an inner wall of the vessel to discharge a high pressure jetted air. In the cylindrical vessel, particles of a toner raw material (i.e., a toner constituent mixture) are suspended by the high pressure jetted air so as to collide with each other and thereby be pulverized. To efficiently perform pulverization while preventing excessive pulverization, the pulverized toner raw material needs to be rapidly fed to a classifier so that desired toner particles having a small particle diameter in a proper desired diameter range can be extracted.
FIG. 3 illustrates a background fluidized bed type pulverization/classification apparatus. Referring to FIG. 3, particles of the toner raw material are fed from a raw material feeder 1 into a cylindrical vessel of a milling chamber 4 by high pressure jetted air fed by an air feeder (not shown) and discharged from plural air nozzles 5. The toner raw material particles collide with each other at crossing points of the jetted air streams generally around the axis 17 of the air nozzles 5 of the milling chamber 4, resulting in pulverization of the toner raw material particles.
The toner raw material particles stay within the cylindrical vessel for a predetermined time while circling therein with this operation. After the pulverization is repeated, the pulverized toner raw material particles are fed by an upward current to a classification rotor 3, which is provided on an upper portion of the cylindrical vessel of milling chamber 4.
The classification rotor 3 classifies the toner raw material particles into particles having a particle diameter in a desired particle diameter range (fine particles that can be discharged using a blower) and coarse particles. The particles having a particle diameter within the desired diameter range can be output from the outlet or exhaust tube 2, and then can be used as a toner in the form of a final product. Coarse particles are fed again by the centrifugal force of the classification rotor 3 back into the cylindrical vessel of milling chamber 4 to be again subjected to the pulverization treatment. By repeating these operations, the toner raw material particles can be converted into particles of a desired size for a final toner product.
If the amount of the particles in the milling chamber 4 are stabilized, the system can operate for a continuous pulverization.
When a final toner powder product having a small particle diameter is manufactured using a fluidized bed type pulverization/classification apparatus and method, the following problems tend to occur:
(1) Continuation of pulverization in a milling chamber is required to prepare the fine particles; and
(2) Coarse particles (having a particle diameter generally not less than about 12 μm) tend to be included in the final toner product. Particularly, the content of such coarse particles in a final toner product prepared by the background fluidized bed pulverization/classification apparatus and method tends to be considerably high.
Japanese Laid-open patent publication No. 11-226443 and Japanese Laid-open patent publication No. 2000-005621 disclose pulverization apparatuses for expedited pulverization. As a device to improve crush efficiency, JP 11-226443 discloses an apparatus having an up-and-down movable drawer bottom so that the surface of the particles that have accumulated on the drawer bottom are held at a current air spout position of a nozzle. However, such a structure does not solve the above-noted problems because nothing is changed in the behavior of the particles from other background art. Further, continuing crushing in the milling chamber in such a device is still necessary to obtain the desired particle size.
JP 2000-005621 discloses an apparatus including a collision member at a center of an axis of the milling chamber. Such an apparatus adopts a particle collision member method. However, in such a device for a collision between the collision member and particles the apparatus needs to intensify the pressure and velocity of air output from air nozzles.
Japanese Laid-open patent publication No. 2004-160371 discloses an apparatus to improve efficiency of pulverizing in which compressed air is jetted from air nozzles to cause a first collision with particles. Further, a second collision member is provided above or below the first colliding position. With such an apparatus collision pulverizing efficiency in a crushing chamber improves. In addition, particles within the desired range can be obtained and pulverizing can be performed with high efficiency. However, in such a publication a shape of the rotor or any benefits achieved by using a rotor of a specific shape is not disclosed.